It is known an electroluminescence device (hereinafter, referred to as an ELD) as an emission type electronic device. As a composing element of an ELD, there are cited an inorganic electroluminescence element (hereinafter, referred to as an inorganic EL element) and an organic electroluminescence element (hereinafter, referred to as an organic EL element). An inorganic EL element has been utilized mainly as a flat light source, however, it requires a high voltage of alternating current to drive an emission element.
On the other hand, an organic electroluminescence element has a structure comprising a light emitting layer containing a light emitting substance being sandwiched with a cathode and an anode, and an exciton is generated by an electron and a hole being injected into the light emitting layer to be recombined. The element emits light by utilizing light release (fluorescence or phosphorescence) at the time of deactivation of the exciton. The light emission is possible at a voltage of approximately a few to a few tens volts, and an organic EL element is attracting attention with respect to such as wide viewing angle and high visual recognition due to a self-emission type as well as space saving and portability due to a completely solid element of a thin layer type.
Moreover, an organic EL device has a distinctive feature of being a surface light, which is different from the main light sources, for example, a light-emitting diode and a cold cathode tube having been conventionally used. As applications which can effectively utilize this property, there are a light source for illumination and a back light of various displays. Especially, it is suitable to use as a back light of a full color liquid crystal display the demand of which has been increased remarkably in recent years.
When an organic EL element is used as a back light of a display or a light source for illumination as described above, there are required to have high light emission efficiency. For improvement in light emission efficiency, it is becoming general to use what is called a host-guest type in which a plurality of materials each respectively have an individual function are mixed for constituting an organic light emitting layer which composes an organic electroluminescence element.
As a production method of an organic electroluminescence element, although there are known a vapor-depositing method, a wet process (for example, a spin coat method, a cast method, an ink-jet method, a spray method and a printing method) (hereinafter, it is also called as a coating method), a production method by a wet process has been attracting attention in recent years for the reason that continuous production is easy and a vacuum process is not needed.
However, an organic EL element produced with a wet process will not exhibit sufficient performance compared with an element produced with a vapor-depositing method. In particular, it has a tendency of increased driving voltage and increased voltage at the time of continued driving.
In the past, the following technology was disclosed as a means which raises the performance of an organic light emitting layer of an organic EL element produced with a wet process those performance is inferior to an element produced with a vapor-depositing method. This technology gave polarization nature to the light source itself, and aimed at improvement in luminosity by applying orientation treatment with a rubbing processing to a nt-conjugated type polymer (refer to Patent document 1).
However, by the method described in this Patent-document 1, there occurred a problem that a film layer was cracked or a film layer was scraped off due to the process by which the formed very thin film was contacted directly.
On the other hand, there was disclosed a technology of reducing the remaining solvent and increasing an adhesive property of an organic light emitting layer to an adjacent layer. This technology contains to apply heat and pressure around the organic light emitting layer with the pressure larger than the atmospheric pressure at the time of formation, or after formation of the organic light emitting layer (for example, Patent document 2). However, it is necessary to heat at a temperature higher than Tg of a composing material of an organic light emitting layer composition in this method. When it is required to use a material having a high Tg, the heating temperature becomes high, and Tg of the substrate has to be made high. There was a problem of difficulty in keeping the flexibility nature enabling to produce an organic electroluminescence element with a curved surface, which is one of the features of an organic electroluminescence element.
Moreover, there was disclosed a technology to improve a light extraction efficiency (it is also called as “an external extraction quantum efficiency”) by forming an organic light emitting layer with a coating method, and then, by stretching it to make oriented the material in the organic light emitting layer (for example, Patent document 3). However, the layer will be destroyed, if the organic light emitting layer is stretched after being coated.
In addition, there was disclosed a technology in which a specific phosphorescence compound is used to make an element to have a specific film density range with a predetermined upper limit and a predetermined lower limit. It is indicated that a voltage increase after a prolonged driving under a constant current and generating of a dark spot are controlled, and the stability of the element with the passage of time under high temperature and high humidity can be improvable (for example, Patent document 4). In the aforesaid Patent document, although the specific numerical value of the film density at the time of using a specific compound is mentioned, it is not considered at all the difference of the film density by changing the production method.